One key issue dominating the hearings on the nomination of Judge Samuel Alito to the U.S. Supreme Court is the scope of presidential power. The issue has been around since the founding of the United States. The terrorist threat has caused it to be raised again.

The scope of presidential power has proved to be one of the most vexing issues running through American history.

Presidential scholars say the issue becomes especially acute in times of war or crisis.

The wording of the U.S. Constitution simply grants the president unspecified "executive powers" and makes him commander in chief of military forces. The powers of Congress are spelled out in more detail than those of the president. As a result, Congress and the president have often clashed over just how much power a president can exercise.

Since September 11th, 2001, President Bush has compared the war on terror to the more traditional forms of nation-to-nation warfare. He has said this gives him wartime powers and the latitude to order the detention of terrorism suspects indefinitely and to conduct electronic surveillance, without court warrants, on U.S. citizens suspected of links to terrorism.

President Bush has said that the constitution, as well as the Congressional authorization to use force against terrorism, grants him the inherent power for actions like the warrantless surveillance.

"Do I have the legal authority to do this? And the answer is, absolutely," said Mr. Bush. "As I mentioned in my remarks, the legal authority is derived from the Constitution, as well as the authorization of force by the United States Congress."

But a new memorandum issued Thursday by the non-partisan Congressional Research Service disagrees, saying the administration's legal justification for the warrantless surveillance is not so clear-cut as the administration claims.

This sort of dispute is why the Alito Supreme Court nomination hearing has focused so intently on presidential powers. Democratic Senator Patrick Leahy of Vermont made his concern about the issue clear on the opening day.

"At a time when this administration seems intent on accumulating unchecked power, Judge Alito's views on executive power are especially important," said Mr. Leahy. "It's important to know if he would serve with judicial independence or as a surrogate for the President nominating him."

Andrew Rudalevige, a professor of political science at Dickenson College and presidential scholar, says the current claims of executive power are rooted in Vice President Dick Cheney's experience in earlier presidential administrations, when Congress moved to limit the chief executive's prerogatives.

"Well, certainly after 9/11 you have a very aggressive claim to unilateral and exclusive executive power by the president," he said. "Actually, the vice president perhaps has been the most vocal about this. He was a staffer in the Nixon and Ford administrations when Congress really was surging ahead and seems to hold a 30-year grudge against the legislative body."

The Vietnam war, the Watergate scandal, and revelations of domestic spying on American citizens caused Congress in the 1970s to pass laws limiting executive authority. Among them was the War Powers Act, which sought to curb the president's authority to wage war without congressional approval.

David Adler, a presidential scholar at Idaho State University, says those attempts to limit presidential powers had little practical effect.

"Well, there were efforts by Congress in the aftermath of the Vietnam war to reassert its power," he noted. "Unfortunately, those legislative efforts amounted to little more than toy handcuffs because in the example of the War Powers Resolution of 1973, Congress did not in fact reclaim its control over war power, which in fact the constitution vests in Congress, but Congress ended up giving the president more authority than the president enjoys under the constitution."

Professor Rudalevige says presidents have worked around congressional limitations through administrative tools such as executive orders and regulations in the vast government bureaucracy. Scholars say President Reagan was able to recover much of the lost presidential power.

Benjamin Kleinerman, a presidential scholar at Virginia Military Institute, says that in recent years Congress has been reluctant to reassert itself and looks to courts, especially the Supreme Court, for help.

"It's decided best when it's, in a way, decided politically, with congress asserting its powers over and against the president, and the president reasserting his powers over and against Congress," he explained. "And these questions have tended to work themselves out politically. Congress now tends to look to the Supreme Court to check the president rather than doing it themselves. In other words, they're hoping that the court will curb the president's power."

In writings early in his career, Judge Alito took a permissive view of presidential powers in intelligence and security matters. But under questioning, he said no president or court is above the law.